The genetic material of killer, an extrachromosomal element in Saccharomyces cerevisiae, consists of several segments of double-stranded RNA. Recent evidence shows that at least one of these segments hybridizes not only to its host DNA but also to the DNA of distantly related fungus. The question remains as to whether fungal genomes, contain sequences related to mycoviral RNA. To answer this question, the RNA of killer factor as well as of other mycoviruses will be hybridized to DNA's extracted from representative fungal species. In each case, the proportion of DNA homologous to mycoviral RNA will be quantitated. Also the relative amount of base-pair mismatching among the various RNA-DNA hybrids will be estimated by thermal denaturation curves.